1. Field of the Invention
The present invention relates to an optical device in which optical rotatory dispersion hardly occurs so that the optical device can be used for forming a vertically aligned liquid-crystal display apparatus wide in viewing angle and excellent in display quality.
2. Description of the Related Art
In a TN type liquid-crystal cell containing liquid-crystal molecules aligned in a direction horizontal to cell substrates with positive dielectric anisotropy, there was a tendency that the quality of black display was deteriorated by light leakage owing to birefringence of liquid-crystal molecules near to the cell substrates. Heretofore, in consideration of the tendency, there was known a liquid-crystal display apparatus including a vertically aligned (VA) liquid-crystal cell containing liquid-crystal cell molecules aligned substantially vertically to cell substrates with positive or negative dielectric anisotropy, polarizers disposed on opposite surfaces of the liquid-crystal cell, and one or two phase retarders exhibiting index anisotropy of nx=ny>nz and interposed between the liquid-crystal cell and either or each of the polarizers. An optical device made of cholestric liquid crystal has been proposed as the phase retarder (Unexamined Japanese Patent Publications No. Sho. 62-210423 and No. Hei. 3-67219).
The phase retarder is provided for compensating for birefringence produced in an oblique viewing azimuth displaced from a frontal direction to prevent light leakage to thereby obtain good black display even in such an oblique viewing direction. That is, the VA liquid-crystal cell transmits light on the basis of substantially vertical alignment of liquid-crystal molecules of the liquid-crystal cell almost without any change of the plane of polarization. Hence, arrangement of the polarizers in the form of crossed-Nicol on opposite surfaces of the cell can achieve light blockade in a frontal direction perpendicular to the liquid-crystal cell when the liquid-crystal cell is inactive, that is, when the liquid-crystal cell is not supplied with any external voltage. Hence, there is a tendency that good black display is formed in the frontal direction but birefringence occurs in an oblique viewing direction. The phase retarder is provided for compensating for such birefringence. In the background-art cholesteric liquid-crystal type phase retarder, there was however a problem that display quality is deteriorated because of variations in the compensating effect.